new-ish to me 68 280SL suddenly no start

[duboy]

Hi all, I am about to get started working on our 68 280SL that was passed down to me from my dad, after he decided to only be interested in golf :-) We've had it in our possession for a while now, just had too many other projects to get through. Since i've had it, i've just changed oil, driven it around, kept it clean. Yesterday, went to go for a drive and no-start. When I would turn they key to position 2, I used to hear a hum, but no longer. And she just cranks, but never starts. The hum was quite pronounced, and i've always remembered hearing it, was this actually the fuel pump? i always thought the noise was coming from under the hood though, not behind me...i should have paid more attention.

When the key is in position 2, i don't feel the pump doing anything either, i'm assuming i should feel it vibrating at a minimum. I checked the fuse, it's good, going to check for spark today, i am assuming spark is good though, she was just fired up and driven 2 weeks ago last and all was fine, and had between 1/4 and 1/3 tank of fuel. Hopefully I can get more trouble shooting going today. Any other ideas?

[Iconic]

If you used to hear the "hum" or "whine" of the fuel pump, and now you don't, this is what you should focus on.
The location where noises come from can be deceptive.
If I remember correctly, you can feel the pump working when it is on, so, once again, this is where you should focus.
The typical failure mode is when it sits around for a long time, it gets gummed up, but this is not the case for you since you recently drove it.
Be careful around fuel !!!

[Jonny B]

An easy check is with the fuses in the fuse panel on the firewall. I have a yearly ritual of rotating the fuses to make sure there is good contact with the retaining clips.

[ctaylor738]

If you know the fuse is good, as in has 12 volts on the output side with the ignition on, and the pump doesn't run, then your fuel pump is probably bad.  The final check would be for power and ground at the pump.

BTW, the metal strips on fuses can crack, and they will look OK, but not function electrically.

[ja17]

Yes, check the fuse first.  If the fuse is good, try testing for power at the pump with a 12 volt test light.  If you have power, try tapping the pump with a wood mallet or plastic hammer. The pumps can get stuck if some debris enters it. A good jolt will often get it spinning again!

[duboy]

Thanks for the info everyone, i tested up to the fuses last night, and i'm getting power through the fuses. Just for good measure i swapped a known working one with the fuel pump fuse, same result.

When i get home today i'll get underneath and test the leads to the pump itself for power. Will update with what i find.

Edit: Arg, just went down to the garage to test the power to the pump. found out 1) have a later style short pump, and 2) i'm guessing someone's jacked with it in the past because the pump cover (secured on with the 2 nuts and 1 bolt) doesn't come out without disconnecting some lines or cutting the metal. It looks like it's been cut already too. Right now, the fuel intake line (from the tank) is threaded through a 1.5" square hole cut into the pump cover. if i want to get the cover off without disconnecting lines, i have to snip about 1" of the cover out. which is totally doable, but...is this a huge no no? as i said, it's already been toyed with.

Oh and there was just enough room for me to tap on it, unfortunately tapping did not bring it back to life....

[ctaylor738]

I personally don't have any qualms about hacking the cover.  Just make stock-looking cuts and put som paint on the raw edges,  But the probability is very high that the pump will need to be replaced so you might as well disconnect the lines.  Suggest you buy a set of fuel line clamps before proceeding.

[duboy]

quick update. i finally got around to replacing the pump. the fuel lines and clamps were in good shape, they must have been touched at some point in the recent history. i was able to re=use them, no corrosion of clamps, or cracking of hoses.

i tested power at the pump ( i finally fenagled the cover around to get to power), and was getting it, so that pretty much confirmed. I drained the tank dry, and did the pump replacement. didn't have any gas cans handy so sometime over the next week or so i'll get a couple and put in at least 3 or 4 gallons and see what happens.

I seem to see a general consensus that if i just leave the key in "on" for maybe 20 or 30 seconds, that's about enough time for the system to prime before i can try a start. that sound about right?

[Jonny B]

The point about leaving the pump run for 20-30 seconds has been discussed on the forum, with some positive results. Just be careful, to no over do this, As you are leaving the ignition system energized which can lead to some potential for damage to the points. All I do is turn the key and let the pump run for a couple of seconds. My anecdotal evidence did not see any affect by the longer "prime" time.

[duboy]

Just an update. She's working well again :-) i needed to "prime" for maybe 5 - 7 seconds, fired right up after. Tomorrow is oil change and fluids checking. So...i'm glad things worked out for the better. Thanks for the advice/answers everyone!